looks pretty good to me. im not a cab grower but ive seen people start with alot less area.
The one thing you probably want to do is replace the back piece of thin compressed wood with a 1/4" piece of plywood just so you can mount fans or other things to it and not have to worry about it falling down, also the back piece absorbs alot of moisture.
check free craigslist! might even find a better one for free.